1. Field of the Invention
The present invention relates to a method of stabilizing the frequency of a semiconductor laser to a predetermined frequency precisely and an apparatus for practicing the method.
2. Description of Related Arts
Light source for semiconductor laser or laser diode whose frequency is stabilized to a specified frequency plays an important role in frequency division multiplexing coherent transmission system or high-resolution optical measurement which makes use of characteristics of light as wave as described in K. Nosu et al.; IEEE, J. Light Wave Technology, Vol. LT-5, pp. 1301-1308 (1987), and Ohkoshi and Kikuchi; "Kohirento Hikari Tsushin Kogaku" (English translation: Coherent Light Communication Engineering), Ohm Co., Tokyo, 1989.
As for the method of stabilizing the frequency of semiconductor laser diodes, there have been known various methods such as a method in which a Fabry Perot resonance oscillator (cf. A. Solberger et al.; IEEE, J. Light Wave Technology, Vol. LT-5, pp. 485-491 (1987), for example.), a method in which use is made of light absorption spectrum of an element such as rubidium (Rb) which appears when energy transition takes place.(cf H. Tsuchida et al.; Japan J. Appl. Phys., Vol. 21, pp. L1-L3 (1982), and the like.
However, these conventional methods have various problems that precision of frequency stabilized is poor, it is difficult to assure stabilization of frequency over a long period of time, and it needs apparatus of a large size as well as that the wavelength region which can be stabilized is limited only to a range of from 0.8 .mu.m to 1.3 .mu.m. Among them the lastly mentioned problem concerning limited range of stabilizable wavelength region is particularly important and explanation will be helpful. That is, in light communication technique put into practice in telephone and data communication and that which is now under development such as frequency division multiplexing coherent transmission system referred to above, the wavelength region mainly used is 1.5 .mu.m wavelength region, more strictly a wavelength region centered at 1.55 .mu.m at which transmission loss of single mode optical fiber is minimum. At the wavelength of 1.55 .mu.m single mode optical fibers show minimum transmission loss (i.., maximum transmission) so that there can be attained maximal elongation of the distance of light transmission via optical fiber system without relays or the length of optical fiber between relays, which is advantageous for long distance light transmission on land or submarine light transmission. Therefore, there is a keen need for developing a light source for semiconductor laser in which frequency is stabilized at a wavelength region of 1.5 .mu.m, particularly at a wavelength near 1.55 .mu.m. At the same time, this light source must be small and highly stabilized from practical viewpoint.
Various methods have heretofore been known in which the frequency of semiconductor laser diode is stabilized in a wavelength region of 1.5 .mu.m. Firstly, there is a method in which there are used optical absorption lines of ammonia (NH.sub.3) molecule which has several optical absorption lines in the wavelength region of 1.5 .mu.m (cf. M. Ohtsu et al.; Japan J. Appl. Phys., Vol. 22, pp 1553-1557 (1983), and T. Yanagawa et al.; Appl. Phys. Lett., Vol. 45, No. 8, pp 826-828 (1984)). In this method, the optical absorption line of ammonia at a wavelength of 1.519 .mu.m (i.e., the strongest optical absorption line) is utilized to stabilize the frequency of distributed feedback laser diode. In this case, there are problems that even at that wavelength (1.519 .mu.m) the intensity of optical absorption by ammonia molecule is low and therefore a long cell as long as 50 cm to 1 m is necessary for the frequency stabilization, which means that the size of apparatus is large, as well as that stability for a long period time is poor. Furthermore, optical absorption lines at wavelengths other than 1.519 .mu.m show much lower intensity of absorption and therefore it is difficult to stabilize the frequency of semiconductor laser diode at a wavelength near 1.55 .mu.m referred to above.
Next, there is known a method of stabilizing the frequency of semiconductor laser diode utilizing optogalvanic effect in which voltage is generated when light with a wavelength of 1.533 .mu.m is irradiated to a hollow cathode lamp enclosing krypton (Kr) (cf. Y. C. Chung et al; Electronics Letters, Vol. 24, pp 1048-1049 (1988)). One problem of this method is that the life time of a hollow cathode lamp is limited to about 500 hours and another problem is that the wavelength which can be stabilized is limited to 1.533 .mu.m.
Recently, a third method has been proposed in which the frequency of distributed feedback laser diode with a wavelength of 1.56 .mu.m is stabilized utilizing optical absorption line of rubidium (Rb) at 0.78 .mu.m after converting the wavelength of a semiconductor laser with a frequency of 1.56 .mu.m by using a wavelength conversion element (such as elements comprised by LiNbO.sub.3, KTP, or LiIO.sub.3) to a wavelength half as long as that of the original wavelength (0.78 .mu.m) (cf. M. Ohtsu et al.; Technical Digest of Conference on Lasers and Electro-Optics, p 52 (1989)). In this method, light output obtained by converting the wavelength of a semiconductor laser with a wavelength of 1.56 .mu.m is weak, e.g., as weak as several picowatts (pW, i.e., 1/10.sup.12 W) and therefore it is necessary to use a ultrahigh sensitive light receptor (e.g., a photomultiplier). Thus, the method has problems in practically acceptable stability and reduction of size of apparatus used.
As the most prominent approach for solving the above-described problems, there has been studied a method in which absorption line of acetylene (C.sub.2 H.sub.2) molecule is utilized (cf. S. Kinugawa et al.; "Detection of C.sub.2 H.sub.2 absorption lines with 1.5 .mu.m DFB lasers", 49th Conference of Japan Applied Physics Society, Preliminary Print, p. 815 (1988)). However, the literature describes only results of measurement on absorption line of acetylene but does not contain idea of frequency stabilization of semiconductor lasers. It reports that acetylene has many intense, sharp absorption lines in a wavelength region of from 1.510 .mu.m to 1.525 .mu.m centering around 1.520 .mu.m, and in a wavelength region of from 1.525 .mu.m to 1.540 .mu.m centering around 1.530 .mu.m.
FIG. 2 is a graph showing optical absorption characteristics of C.sub.2 H.sub.2 molecule measured by the present inventors. Although acetylene molecule has a lot of intense, sharp absorption lines in a wavelength region of from 1.51 .mu.m to 1.54 .mu.m, the intensity of absorption of absorption lines is low in a wavelength region of longer than 1.54 .mu.m. Therefore, it is very difficult to maintain optical absorption intense enough to stabilize the frequency of semiconductor laser at 1.55 .mu.m which is practically important as described above. For example, when a cell enclosing acetylene gas at 10 Torr is used, the cell must be 1 m long in order to maintain intensity of absorption at 1.541 .mu.m at a level of 20%, with the result that it is difficult to reduce the size of apparatus to be used and achieve high degree of stabilization. Therefore, even with a method utilizing absorption lines of acetylene molecule, it is very difficult to provide practically acceptable light source for semiconductor lasers which can give stabilized frequency at a wavelength near 1.55 .mu.m.